Organic electroluminescent (EL) devices are generally composed of three layers of organic materials sandwiched between transparent and metallic electrodes, the three layers including an electron transporting layer, an emissive layer and a hole transporting layer. Organic EL devices are attractive owing to the requirement for low driving voltage and the potential application to full color flat emissive displays. Though significant lifetime has been achieved in the prior art (See U.S. Pat. No. 4,720,432), further improvement is needed for applications where high brightness is required. Among other things, the lifetime of an organic EL device is affected by the stability of both the bulk morphology of the hole transporting materials and the interface between the hole and electron transporting layers when the organic EL device is under bias.
Several schemes have been proposed to address the problem of bulk morphology stability of the hole transporting materials in an organic EL device: one being a double-layer hole transporting configuration (see U.S. Pat. No. 5,256,945); and another being usage of materials of high glass transition temperatures (U.S. Pat. No. 5,061,569).
It is a purpose of this invention to address the problem of the interface stability between heterojunctions in an organic EL device.
It is another purpose of this invention to provide a new method to enhance the thermal stability of the organic electroluminescent medium of an organic EL device.
It is a further purpose of this invention to provide an organic electroluminescent device for displays with improved reliability.